The flooding of a damaged ship is a time-dependent process that is significantly affected by the non-watertight structures inside the watertight compartments. For certain ship types, like passenger ships, such structures form a complex internal subdivision. Time-domain simulation is the most realistic approach to calculate progressive flooding in damage stability analyses, but it is necessary to use a simplified method for modelling the leakage and possible collapse of the non-watertight structures. This paper presents unique full-scale tests and advanced finite element analyses, conducted to determine the leakage and collapse characteristics of various typical non-watertight structures, when subjected to water pressure. The obtained results are carefully analysed, and a simplified method for modelling the leakage of closed doors for time-domain flooding simulation is presented. For all tested doors leakage started practically immediately when immersed. Various deformation and collapse mechanisms were observed, and often the leakage increased with larger pressure head due to the deformation of the door. The collapse pressure heads varied between 1.0 m and 3.5 m; the cold room door having the largest value. Guideline values for typical non-watertight doors were derived based on the obtained results.